What is the .UK WhoIs opt out option?

This article applies to the use of WhoIs Privacy with 123-reg Domain names

The WhoIs opt out option applies only to .UK based domain names. It hides some of the details found on the WhoIs database, although it is not as effective at WhoIs Privacy. Instead of hiding all of your details, the WhoIs opt out only hides your address details, and not the registrant name.